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Thermodynamics Notes12

Thermodynamics Notes12 - ME 311 FALL 2007 CHAPTER 5 The 2nd...

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ME 311 THERMODYNAMICS S. Masutani FALL 2007 1 CHAPTER 5 The 2 nd Law The direction of processes The first law (COE) is applied to confirm that a postulated process or event does not violate the principle that energy cannot be created nor destroyed. Alternatively, this principle is applied to determine unknown values of energy transfers to or from a selected system on the basis of known magnitudes of energy inflows and outflows. By itself, the first law provides a first-level screening of processes or events. Those which satisfy the energy balance are candidates for an additional check based upon the 2 nd Law of Thermodynamics. The Second Law identifies the allowed direction of a process or event. Examples: 1. 2H 2 + O 2 2H 2 O react spontaneously ; what about the reverse reaction? 2. A falling mass strikes the ground and warms up. What about the reverse process? Consider: Adiabatic vessel State A: flywheel spinning; system cool State B: flywheel stationary; system warm Collisions transfer of energy from flywheel to gas molecules; wheel slows down; gas spins in box. Random motions “extract” energy of the organized swirling motion of the gas Æ wheel and gas bulk (organized) motion goes to 0. Finally, all energy resides in random motions of the gas and wheel molecules (gas and wheel become warmer). Energy balance: (U + KE) A = U B initial final energy energy This follows intuition. Next , postulate a process beginning with state B and ending with state A.
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ME 311 THERMODYNAMICS S. Masutani FALL 2007 2 The energy balance for this process is U B = (U + KE) A initial final energy energy The 1 st Law expressions for both processes are identical. Our intuition, or experience, tells us, however, that the 2 nd process will never happen. The COE principle is unable to identify that such a process is impossible. The 1 st Law is insensitive to the direction of the process. State A: energy is highly organized which makes it possible easily to extract this energy in the form of useful work, e.g., attach a generator to the spinning flywheel. State B: energy is microscopically disorganized. It is difficult to extract energy as useful work. In the course of the “randomization” of organized energy we have lost some ability to do useful work. What has been produced? Answer: a higher state of microscopic (i.e., molecular) chaos. Whenever molecular chaos is produced, the ability to do useful work is reduced. Entropy is a property of matter that quantifies the randomness or disorder that exists at the microscopic level. The concept of entropy is as central to the 2 nd Law as energy is to the First Law. The natural order of things calls for entropy to be produced by all processes. Associated with this production of molecular disorder is a loss of ability to do useful work. Energy is degraded (or transformed) to a less useful form. The 2 nd Law of thermodynamics observes that: Entropy can be produced but never destroyed. Consider entropy to be a measure of our uncertainty about the microscopic state of matter.
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Thermodynamics Notes12 - ME 311 FALL 2007 CHAPTER 5 The 2nd...

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